Planographic printing plate



plate.

Un fid States t t0,.

Bernard Cohn, Brooklyn, N.Y., assignor to Polychrome gorioration, Yonkers, N.Y., a corporation of New No Drawing. Application May 3, 1954 Serial No. 427,381

6 Claims. (Cl. 96--75) My invention relates to a new and improved presensitized planographic printing plate and a new and improved method of making such a plate, and is a continuation-inpart of my co-pending application, Serial No. 408,291, filed February 4, 1954.

The art of planographic printing depends upon the immiscibility of grease and water and upon the preferential retention of a greasy image-forming substance by the image areas, and a similar retention of an aqueous dampening fluid by the non-image areas. When a greasy image is imprinted upon a suitable surface, and the entire surface is then moistened with an aqueous solution, the image areas will repel the water, and the non-image areas will retain the water. Upon subsequent application of greasy ink, the image portions retain the ink, whereas the moist, non-image areas repel it. The image is then transferred to paper or cloth, via an intermediary, so called off-set cylinder, which is necessary to prevent mirror-image printing.

In the case of pro-sensitized plates, a plate with a light- .sensitized coating is exposed to light through a negative, and the image portion of the plate becomes hardened and made insoluble in water by the action of the light. The unexposed light-sensitive coating is then removed by a desensitizing solution leaving a water-receptive undercoating as the non-image area.

It has been known to use, as the base material in a pre-sensitized plate, either a waterproofed paper or a flexible sheet ofmetal. Typical metals which may be used are aluminum, steel, zinc, magnesium, chromium, and copper.

Because, paper is relatively coarse-grained and has a tendency to stretch, a plate having a metallic base, will give finer reproduction and longer service than will a paper-base. plate.. When it is desired to coat a metallic plate with a light-sensitive material, however, it is necessary to provide the metal with a surface to which the material may adhere, since it will not adhere to the bare metal. This may be accomplished by coating the metal with a hard siliceous layer which provides an adhesive and a water-receptive undercoating for the light-sensitive .material. The present invention relates to this type of have-been used in plates of this type.

Another problem which arises, particularly in the use of pre-sensitized plates in half-tone work results from the 2 prints obtained from such plates. This phenomenon is known as halation.

I have found that by mechanically graining the plate and then using a solution of a salt of an alkaline earth metal as the hardening agent for the siliceous layer, the press life of the plate may be greatly increased, while, at the same time, the susceptibility of the plates to halation is substantially reduced.

Accordingly, it is an object of my invention to provide a pre-sensitized plate having a longer press life.

Another object of my invention is to provide a presensitized plate which is cleaner running.

A further object is to provide a pre-sensitized plate which will not only run longer but will provide an image which is free from the effects of halation.

These and other objects of my invention will be more clearly understood from the description which follows:

Essentially the invention involves the combination of graining of the surface of the metal by mechanical means and the use of a solution of an alkaline earth metal salt as the hardening agent for the siliceous layer.

In another, and still more specific embodiment, my irivention involves the preparation of an aluminum-base pre-sensitized plate by sandblasting the aluminum, prior to the hardening of the siliceous layer through the use of a calcium nitrate solution.

I have found that a plate having a longer press life, and relatively free from halation may be produced by mechanically graining the plate before providing it with the hard siliceous layer and the light-sensitive coating.

The mechanical means used to grain the metallic plate, are wellknown to the art and may comprise, for example, rubbing with an abrasive, sandblasting, or wire brushing. I particularly prefer to use sandblasting which can be more easily controlled to dent, rather than cut into the surface of the metal.

The graining of the metallic surface in this fashion has been found to produce, in a pre-sensitized plate of the type referred to, herein, and in my co-pending application Serial No. 408,291, filed February 4, 1954, the ability to increase both the quality and quantity of reproductionin a manner not heretofore realized.

In sandblasting the aluminum or other metal, the sand is forced through a nozzle under high compression for a length of time such that the surface is completely grained. This can be easily determined by visual examination, preferably with the use of a magnifying glass. The metallic surface should have the appearance of being dented rather than abraded. Obviously the type and coarseness of the sand may be varied depending on the type of metal and the degree of roughness desired in the metal surface. Usually the use of 50 to 70 mesh sand is satisfactory for aluminum plates.

After the mechanical graining treatment, the plate 'is given an alkaline etch carried out between 50 C. and 70 C. and preferably between 55 C. and 58 C. Following the alkaline treatment the plate is rinsed in water at room temperature from 15 to 30 seconds to remove the alkaline residue, after which the impurities arezremoved from the surface by a desmutting process. Typical desmutting solutions are either 50% nitric acid or a mixture of 3% chromic, sulfuric and phosphoric acids. Then the plate is again rinsed and a siliceous layer is formed by dipping the plate in a sodium silicate solution at elevated temperatures, preferably above C.

This is .followedby the novel step of hardeningmthe siliceous layer by treating the plate with a soluble salt of an alkaline earth metal, i.e. calcium, barium, strontium, or magnesium, thereby converting the siliceous layer to the corresponding alkaline earth silicate.

In a more specific and preferred embodiment, calcium nitrate may be used. It is used preferably at room .perature in running water.

temperature but not above 40 C. The concentration of calcium nitrate should generally be at least 3% by weight, a solution has 'been found to be quite satisfactory when the immersion time is about 2 minutes. However, the concentration may be varied depending ppon the desired contact time, and even concentrations below 3% would be operative if the immersion period is long enough. It is usually desirable, however, to have the contact time between 1 /2 and 5 minutes. The same consideration of concentration and contact time apply when using the other alkaline earth salts, although magnesium salts would generally require somewhat higher concentrations by weight because of their'lower molecular weights.

If desired, after rinsing, the plate may again be immersed in the silicate bath and the hardening treatment repeated. This may be desirable as two thin coatings of silicate will tend to give a longer running and cleaner running plate than would be obtained with a single thicker layer, since a thick silicate layer does not allow as much penetration of the calcium hardener. The second silicate coating may be applied at any temperature between the limits of 20 C. and 100 C. Ordinarily, the silicate coating will not exceed 1 micron in thickness.

After the final calcium nitrate treatment and final rinse, the plate is contacted with the sensitizer which may be a diazo type material. A typical diazo compound which may be used is the condensation product of paraformaldehyde and paradiazodiphenylamine. The sensitizer may comprise 1% diazo and .1% of a solution of saponin. Another example of a sensitizer would be the sodium salt 2,7 anthraquinone disulfonate.

Following is a specific example for a method of preparing an improved aluminum plate for use in combination with the mechanical graining step and the silicate hardening treatment. It should be understood that no novelty is claimed for the mechanical graining step per se but only in combination with the method of preparing the hardened silicate presensitized plate herein described.

Specific example An aluminum plate 10" by and .0055" thick was sandblasted by blowing with a 70 mesh English sand until an examination thereof revealed a thoroughly grained surface.

The plate was then immersed in an alkaline solution of .6% of sodium hydroxide, 1.2% of tetrasodium pyrophosphate, and 1.2% of sodium tripolyphosphate, for one minute at 55 C. The alkaline residue was then removed by a rinsing for 30 seconds in water at room temperature and then in a desmutting solution of 3% by weight of CrO 3% by volume of H SO and 3% by volume of H PO Then it was rinsed for three minutes in running water and immersed in a silicate bath consisting of (Na O:SiO =1:3.22).

This solution was maintained at 85 C. Then the plate was removed and rinsed for 3 minutes at room tem- It was then hardened by immersion in a 5% solution of calcium nitrate at 25 C. for 2 minutes. The plate was then rinsed again in running water at room temperature for 3 minutes and then it was immersed into the silicate bath again as hereinbefore illustrated. After the final calcium nitrate treatment, it was rinsed again for 3 minutes in acidified water. The pH was maintained at about 4 with phosphoric acid and dried under infrared lamps. Then the plate was coated with an aqueous solution of 1% diazo and 0.1% of a 10% solution of saponin. The diazo :used was the condensation product of paraformaldehyde and paradiazodiphenylamiue (4 parts of formaldehyde to 30 parts of paradiazodiphenylamine by weight). The plate was coated on both sides, retaining about 0.25 cc. of diazo solution per side. After drying, the plate was ready for use.

The plates, prepared in the manner herein described have been found to produce 60,000 to 80,000 copies of printed matter Without loss of quality of reproduction.

Moreover, these prints, when examined, are found to be free from the effects of halation, i.e. they contain no darkened areas or halos due to almost microscopic impurities such as dust particles.

It is to be understood, that my process is not limited to the specific example herein described, but is to include the many variations thereof which would be obvious to those skilled in the art. Therefore, although I have described one preferred form of my invention, I do not wish to be limited except as set forth in the appended claims.

1 claim:

1. The method of preparing a presensitized, aluminum -planographic printing plate which comprises mechanically graining the aluminum surface by abrasive action, coating the grained surface with a siliceous layer by treatment with an alkali silicate solution, and hardening said siliceous layer by treatment with an aqueous solution containing at least 3 percent, by weight, of an alkaline earth salt.

2. The method of claim 1 in which the aluminum surface is grained by sandblasting.

3. The method of claim 1 in which the aluminum surface is grained by sandblasting and the plate is coated with a light-sensitive, diazo compound.

4. The method of preparing a presensitized, aluminum planographic printing plate which comprises mechanically graining the aluminum surface by abrasive action, coating the grained surface with a siliceous layer by treatment with an alkali silicate solution, and hardening said siliceous layer by treatment with an aqueous solution containing at least 3 percent, by weight, of calcium nitrate.

5. The method of preparing a presensitized, aluminum planographic printing plate which comprises mechanically graining the aluminum surface by abrasive action, coating the grained surface with a siliceous layer by treatment with an alkali silicate solution, and hardening said siliceous layer by treatment with an aqueous solution containing at least 3 percent, by weight, of an alkaline earth salt, and coating the plate with a light-sensitive, diazo compound.

6. A planographic printing plate made by the method of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Lange: Handbook of Chemistry, 8th ed., 1952, Handbook Pub. Co., Sandusky, Ohio, pages 786-789. (Copy in Sci. Libr.) 

4. THE METHOD OF PREPARING A PRESENSITIZED, ALUMINUM PLANOGRAPHIC PRINTING PLATE WHICH COMPRISES MECHANICALLY GRAINING THE ALUMINUM SURFACE BY ABRASIVE ACTION, COATING THE GRAINED SURFACE WITH A SILICEOUS LAYER BY TREATMENT WITH ALKALI SILICATE SOLUTION, AND HARDENING SAID SILICEOUS LAYER BY TREATMENT WITH AQUEOUS SOLUTION CONTAINING AT LEAST 3 PERCENT, BY WEIGHT, OF CALCIUM NITRATE. 